As it is well known, the systems of imaging medical diagnostic testing such as echography and ultrasonography, which are widely used in the surgical and radiological fields, use ultrasonic or ultrasound waves and are based on the principle of the transmission of the ultrasounds and of the echo emission.
A diagnostic apparatus or an ultrasound imaging system includes three parts, as shown in FIG. 1:                a probe 2 that is maintained in direct contact with the skin of the subject and that includes at least one transducer element 3 suitable for transmitting and for receiving an ultrasound signal;        a electronic system 4 that drives the transducer element 3 for generating pulse ultrasound signals to be transmitted and that receives a return echo signal at the probe of this pulse, in consequence treating the echo signal received; and        a displaying system 5 of a corresponding echographic image processed by a computer and shown on a monitor starting from the echo signal received by the probe.        
The ultrasounds that are generally used are between approximately 1 and 20 MHz and can be generated by a piezoceramic crystal inserted in the probe.
In FIG. 1 the probe 2, in a simplified scheme, includes a linear array 6, or 1D array, of transducer elements 3.
In particular, the electronic system 4 includes a transmission channel that applies a voltage signal to each transducer element 3 of the probe 2 and that includes a beamformer device, a pulse generator, and a high-voltage transmitting circuit placed in cascade with each other. The transducer element 3 receives the electronic voltage signal and generates, in response to the voltage signal, a respective ultrasound pressure signal or acoustic signal.
The electronic system 4 also includes a reception channel Cn associated through a TR switch with said transducer elements 3, the TR switch allowing in particular to protect the reception part, the echo signal received being a low-voltage signal while the transmission signal is a high-voltage signal. The reception channel Cn is repeated for each transducer element 3 and includes, in cascade, a Low Noise Amplifier (LNA), a variable-gain block TGC suitably controlled by means of a controller, a Programmable Gain Amplifier (PGA), an analog-to-digital converter (ADC), an adder, and a reception beamformer device that re-phases the digitalized echo signals received by the probe.
In the linear array probe 6, the electronic circuitry is maintained outside the probe itself, and all the transducer elements 3 are each coupled to the computer of the displaying system 5 through a respective coaxial cable 8, as shown in FIG. 2.
The transducer elements 3 and the corresponding coaxial cables 8 are of a number N between approximately 100 and 200, generally equal to about 130. The coaxial cables 8 are provided with the length of about two meters.
The echo signals coming from a single point A are received by the transducer elements 3 in a delayed way with respect to each other, with a different delay in relation to the distance that the echo signal must cover to reach a respective transducer element.
For a correct reconstruction of the image, a delay compensation between the acoustic signals sent by the transducer elements 3 is performed.
As shown in FIG. 3, the peak of the echo signal received shows a phase shift at the time level which is in relation to the angle α subtended with respect to the transducer element having the shortest path.
The acoustic signals are suitably digitalized and re-phased in the digital circuits of the beamformer device 7 so as to reconstruct the image correctly.
For improving the performances and obtaining 3D echographic images, it is known to use matrix probes 2, which include a matrix 10 of transducer elements 3, shown for example in FIG. 4, with a number N′ of transducer elements 3 between approximately 1000 and 8000. Such a matrix probe 2 allows a compensation of the echo signals received on both the axes of the matrix and allows a display of 3D images.
As schematically shown in FIGS. 5A and 5B, the matrix probe 2 can be realized with a multilayer semiconductor structure wherein the associated layers are coupled through conductive vias, realizing, in correspondence with a surface of the structure, thousands of transducer elements 3 electrically coupled to each other and in correspondence with the opposed surface to the electronic circuitry. This matrix probe 2 improves the performances by allowing a compensation of the echo signals on both the axes of the matrix so as to obtain a 3D image.
The transducer elements 3 of the matrix 10 are suitably grouped in sub-matrixes 21, rectangular or squared of sizes, for example equal to (2×2), (3×3) or (6×6), so as to pre-process the echo signals received and reduce to only about one hundred the number of coaxial cables 8 sent to the displaying system 5.
Coming back to FIG. 4, the matrix probe 2 includes a portion of circuitry including in turn for each transducer element 3 a respective reception channel Cn. Each reception channel Cn includes a TR switch SW and a low noise amplifier LNA placed in cascade with each other and coupled to a beamformer device 12.
The beamformer device 12, as shown in FIG. 6, includes a plurality of storage cells 14 arranged in second re-phasing matrices 15, each second re-phasing matrix 15 being associated with a corresponding sub-matrix 11 of transducer elements 3. Each row Ri of the second re-phasing matrix 15 is associated with one of said transducer elements 3 through the reception channel Cn.
Each cell 14 of the beamformer device 12, shown in FIG. 7, includes a capacitor C, which is interposed between a ground terminal coupled to a ground gnd and an input terminal In, which is coupled to the output of the low noise amplifier with variable gain and which receives the acoustic signal through a writing switch S1 driven by a writing signal “write” (or “wr”). A reading switch S2 driven by a reading signal “read” (or “rd”) allows generating an output signal Out on the basis of the charge value stored in the capacitor C.
According to the embodiment of FIG. 6, a selector 18 generates the writing signal wr and the reading signal rd for driving respectively the writing switch S1 and the reading switch S2 of each cell 14. The reading switch S2 of each storage cell 14 is coupled to a single buffer 16 through a respective column terminal Tri.
In a conventional system there is a writing step by column of the second re-phasing matrix 15 of storage cells 14. In particular, on the basis of the delay of the echo signal received in the rows of the second re-phasing matrix, a representation of the echo signal is stored in cells of successive columns. The conventional system then provides a reading step that allows generating the output signal Vout corresponding to a same echo signal received by activating suitable storage cells 14 which belong to successive columns Coi of the second re-phasing matrix 15, in a predetermined way.
An embodiment shown in FIGS. 8-11 represents four channels C1-C4 and four focusing points, indicated with A, B, C and D while the numbers indicate the successive sequences of time.
At time 1, shown in FIG. 8, the first and the second transducer element 3 receive the echo signal from point A and generate in the respective channel, C1 and C2, an acoustic signal which is first amplified and then stored in the cells 14 of the first column Co1 of the second re-phasing matrix 15. This first column Co1 is completely activated by the selector 18 at the time 1 through the writing signal wr.
At time 2, shown in FIG. 9, with a certain delay, the third and the fourth transducer element 3 receive the echo signal from point A, and generate in the respective channel, C3 and C4, an acoustic signal which is stored by the cells 14 indicated with 2A of the second column Coi completely activated by the selector 18. Simultaneously, the first and the second transducer 3 receive the echo signal from point B and generate the acoustic signal which is stored in the cells 14 activated and indicated with 2B of the second column Coi.
At time 3, shown in FIG. 10, the third and the fourth transducer elements 3 receive the echo signal from point B and generate a respective acoustic signal which is stored in the cells 14 indicated with 2B of the third column Coi completely activated by the selector 18. Simultaneously, the first and the second transducer elements 3 receive the echo signal from point C and generate the acoustic signal, which is stored in the cells 14 indicated with 3C of the third column Coi.
Similarly, at time 4, shown in FIG. 11, the third and the fourth transducer elements 3 generate the acoustic signal corresponding to the echo signal of point C, which is stored in the respective cells 14 of the fourth column Coi and indicated with 4C, where the column is activated by the selector 18. Simultaneously, the first and the second transducer elements 3 receive the echo signal from point D, and they generate the corresponding acoustic signal which is stored in the cells 14 indicated with 4D of the fourth column Coi activated, and so on.
The cells 14 indicated with 1A and 2A are then read, respectively in the first column Co1 and in the second column Co2, for representing the signal generated by point A, and the cells 14 indicated with 2B and 3B respectively in the second column Coi and in the third column Coi are read for representing to the signal generated by point B, and so on.
This system, although advantageous under several aspects, has the drawback of a rather laborious and complex reading step, which includes activating cells that belong to two or more columns of the second re-phasing matrix for representing the echo signals coming from the same focusing points. The activating cells of different columns indicates that the output terminal of each cell is coupled to a single column terminal Tri. Since the second re-phasing matrix 15 occupies a certain area in an integrated circuit, the column terminal Tri is constituted by a rather long metallization wire, and this indicates a significant parasitic capacitance associated with said terminal. The parasitic capacitance may worsen the signal-to-noise ratio, thus possibly degrading the performance of the system.